<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script src="./jquery.js"></script>
    <style>
        main {
            margin: 0px;
            padding: 0px;
        }
        
        #demo {
            width: 1000px;
            height: 200px;
            box-sizing: border-box;
        }
        
        #demo>ul {
            width: 100%;
            height: 100%;
        }
        
        #demo>ul>li {
            list-style: none;
            float: left;
            width: 200px;
            height: 100%;
            overflow: hidden
        }
    </style>
</head>

<body>
    <div id="demo">
        <ul>
            <li>
                <a href="#"></a><img src="./background/bj1.jpg" alt=""></li>
            <li>
                <a href="#"></a><img src="./background/bj2.jpg" alt=""></li>
            <li>
                <a href="#"></a><img src="./background/bj3.jpg" alt=""></li>
            <li>
                <a href="#"></a><img src="./background/bj4.jpg" alt=""></li>
            <li>
                <a href="#"></a><img src="./background/bj5.jpg" alt=""></li>
        </ul>
    </div>
</body>
<script>
    $(function() {
        $('#demo>ul>li').mouseenter(function() {
            $("#demo>ul>li").css("width", "100px");
            $(this).css("width", "600px");
        }).mouseleave(function() {
            $("#demo>ul>li").css("width", "200px")
        })
    })
</script>

</html>